1. Field of the Invention
The present invention relates to improvements in non-aqueous electrolyte secondary batteries, such as lithium-ion batteries, and more particularly to, for example, a battery structure that is excellent in cycle performance and storage performance under high-temperature conditions and that exhibits high reliability even with a high power battery design.
2. Description of Related Art
Mobile information terminal devices such as mobile telephones, notebook computers, and PDAs have become smaller and lighter at a rapid pace in recent years. This has led to a demand for higher capacity batteries as the drive power source for the mobile information terminal devices. With their high energy density and high capacity, lithium-ion batteries that perform charge and discharge by transferring lithium ions between the positive and negative electrodes have been widely used as the driving power sources for the mobile information terminal devices.
The mobile information terminal devices tend to have higher power consumption according to the functions of the devices, such as a moving picture playing function and gaming functions. It is strongly desired that the lithium-ion batteries that are the drive power source for the devices have further higher capacities and higher performance in order to achieve longer battery life and improved output power.
Under these circumstances, research and development efforts to provide lithium-ion batteries with higher capacities have been underway. These efforts center around attempts to reduce the thickness of the battery can, the separator, or positive and negative electrode current collectors (e.g., aluminum foils or copper foils), as disclosed in Japanese Published Unexamined Patent Application No. 2002-141042, which are not involved in the power generating element, as well as attempts to increase the filling density of active materials (improvements in electrode filling density). These techniques, however, seem to be approaching their limits, and fundamental improvements such as finding alternative materials have become necessary to achieve a greater capacity in lithium-ion batteries. Nevertheless, regarding attempts to increase the battery capacity through alternative positive and negative electrode active materials, there are few candidate materials for positive electrode active materials that are comparable or superior to the state-of-the-art lithium cobalt oxide in terms of capacity and performance, although alloy-based negative electrodes with Si, Sn, etc. as negative electrode active materials appear to be promising.
Under these circumstances, we have developed a battery with an increased capacity by raising the end-of-charge voltage of the battery, using lithium cobalt oxide as the positive electrode active material, from the currently common 4.2 V to a higher region to increase the utilization depth (charge depth). The reason why such an increase in the utilization depth can achieve a higher battery capacity may be briefly explained as follows. The theoretical capacity of lithium cobalt oxide is about 273 mAh/g, but a battery rated at 4.2 V (the battery with an end-of-charge voltage of 4.2 V) utilizes only up to about 160 mAh/g, which means that it is possible to increase the battery capacity up to about 200 mAh/g by raising the end-of-charge voltage to 4.4 V. Raising the end-of-charge voltage to 4.4 V in this way accomplishes about a 10% increase in the overall battery capacity.
When lithium cobalt oxide is used at a high voltage as described above, the oxidation power of the charged positive electrode active material increases. Consequently, decomposition of the electrolyte solution is accelerated, and moreover, the delithiated positive electrode active material itself loses stability of the crystal structure. Accordingly, most important issues to be resolved have been the cycle life deterioration and the performance deterioration during storage due to crystal disintegration. We have already found that addition of zirconia, aluminum, or magnesium to lithium cobalt oxide can achieve comparable performance to the 4.2 V battery even at a higher voltage under room temperature conditions. However, as recent mobile devices require higher power consumption, it is essential to ensure battery performance under high-temperature operating conditions so that the battery can withstand continuous operation in high temperature environments. For this reason, there is an imminent need to develop technology that can ensure sufficient battery reliability even under high temperature conditions, not just under room temperature conditions.
It has been found that the positive electrode of a battery with an elevated end-of-charge voltage loses stability of the crystal structure and shows a considerable battery performance deterioration especially at high temperature. Although the details are not yet clear, there are indications of decomposition products of the electrolyte solution and dissolved elements from the positive electrode active material (dissolved cobalt in the case of using lithium cobalt oxide) as far as we can see from the results of an analysis, and it is believed that these products and elements are the primary causes of the deterioration in cycle performance and storage characteristics under high temperature conditions.
In particular, in the battery system that employs a positive electrode active material composed of lithium cobalt oxide, lithium manganese oxide, lithium-nickel-cobalt-manganese composite oxide, or the like, high temperature storage causes the following problems. When stored at high temperature, cobalt or manganese dissociates into ions and dissolves away from the positive electrode, and subsequently, these elements deposit on the negative electrode and the separator as they are reduced at the negative electrode. This results in an increase in the battery internal resistance and the resulting capacity deterioration. Furthermore, when the end-of-charge voltage of the lithium-ion battery is raised as described above, the instability of the crystal structure is worsened, and the foregoing problems are exacerbated, so the foregoing phenomena tend to occur even at a temperature of about 50° C., where the battery rated at 4.2 V does not cause such problems. Moreover, these problems tend to worsen when a separator with a small film thickness and a low porosity is used.
For example, with a battery rated at 4.4 V that uses a lithium cobalt oxide positive electrode active material and a graphite negative electrode active material, a storage test (test conditions: end-of-charge voltage 4.4 V, storage temperature 60° C., storage duration 5 days) showed that the remaining capacity after the storage deteriorated considerably, in some cases as low as about zero. Following the disassembly of the tested battery, a large amount of cobalt was found in the negative electrode and the separator. Therefore, it is believed that the elemental cobalt that has dissolved away from the positive electrode accelerated the deterioration. The valency of the positive electrode active material that has a layered structure, such as lithium cobalt oxide, increases by the extraction of lithium ions. However, since tetravalent cobalt is unstable, the crystal structure thereof is unstable and tends to change into a more stable structure. This is believed to cause the cobalt ions to easily dissolve away from the crystals. It is also known that when a spinel-type lithium manganese oxide is used as the positive electrode active material as well, trivalent manganese becomes non-uniform, and dissolves away from the positive electrode as bivalent ions, causing the same problems as in the case of using lithium cobalt oxide as the positive electrode active material.
As described above, when the charged positive electrode active material has an unstable structure, performance deterioration during storage and cycle life degradation under high temperature conditions tend to be more evident. It is also known that this tendency is more evident when the filling density of the positive electrode active material layer is higher, so the problems are more serious in a battery with a high capacity design. It should be noted that even the physical properties of the separator, not just the negative electrode, are involved because, for example, by-products of the reactions produced from the positive and negative electrodes migrate through the separator to the opposite electrodes, further causing secondary reactions. Thus, it is believed that ion mobility and migration distance within the separator are greatly involved.
To overcome such problems, attempts have been made to prevent cobalt or the like from dissolving away from the positive electrode by, for example, physically coating the surface of the positive electrode active material particles with an inorganic substance, or by chemically coating the surface of the positive electrode active material particles with an organic substance, for example, a polymer of a cyclic aromatic hydrocarbon, or of a biphenyl or the like. However, in the case of a physical coating, since the positive electrode active material more or less expands and shrinks repeatedly during charge-discharge cycling, the advantageous effect resulting from the coating may be lost. On the other hand, in the case of a chemical coating, it is difficult to control the thickness of the coating film. If the thickness of the polymer layer is too large, the internal resistance of the battery increases, making it difficult to attain desired performance, and as a result, the battery capacity reduces. Moreover, there remains an issue that it is difficult to coat entire particles, limiting the advantageous effect resulting from the coating. Thus, there is a need for an alternative technique to the coating methods.